It is sometimes desirable to deliver a fluid using a pulsatile fluid flow or series of pulses. For example, some medication delivery systems which utilize a series of pulsatile fluid pulses to deliver medication, are known in the art. Medication delivery systems may be used to deliver pain control medication and other medications intraoperatively or post-operatively, subcutaneously, and percutaneously to a patient after a surgical, or some other medical, procedure.
For example, U.S. Pat. No. 5,807,075 to Jacobsen et al. discloses a conventional medication delivery system that includes a base housing and a cassette. The base housing of the '075 patent houses electronic components, such as an electric motor, a power source, and an electronic controller, and the cassette of the '075 patent interacts with a supply of the medication to deliver the medication to the patient.
A further example of a conventional medication delivery system is disclosed in U.S. Pat. No. 4,650,469 to Berg et al. This patent discloses a medication delivery system that includes a control module and a reservoir module removably connected to the control module. The control module includes a pump mechanism, valves, a power source, electronic controls, and the like, and the reservoir module includes a container that supplies the medication to be delivered to the patient.
It is known to use an electric motor in such medication delivery systems, where each revolution or cycle of the motor delivers a preset amount of medication. Such systems are known as positive displacement systems. In such systems, the flow of medication is not pressurized unless it meets a restriction.
Generally, conventional medication delivery systems provide a flow of medication through an output tube which then is delivered to the patient, as required. However in some procedures, medication is required at two locations with respect to the patient, for example, breast augmentation or reconstruction. Another such procedure where medication delivery is desirable at two sites is an autologous graft procedure where it is desirable to deliver medication at both the graft and the donor sites. If the medication provided by the delivery system is pumped through a “Y” connection, then the medication will not be delivered to each site or location evenly for several reasons. First, unequal pressure at the two infusion sites due to elevation or intracompartmental pressure sets up a siphon where flow occurs from the higher pressure side to the lower pressure side in the period between pulses. Furthermore, if the flow of medication on one side of the “Y” has a greater restriction than on the other side, back pressure may reduce or stop the flow of medication on that side. This is undesirable.
One solution would be to provide a check valve in each leg after the “Y” connection. This solution presents several problems, namely, there is a time delay added by the opening and closing of the check valve and differences in manufacturing tolerances contributing to the delay may also lead to uneven delivery of the medication. Furthermore, most check valves restrict flow when open, and unequal or uncontrollable variations in this restriction would lead to unequal flow.
Another solution would be to provide a large fluid resistor (small orifice) in each leg. Correctly sizing this orifice would cause the pressure to rise substantially higher than the downstream pressure differences. This pressure could be driven up over several pulses. If the pressure remained higher than the highest downstream pressure, no backflow due to siphoning could occur. Furthermore, the difference in the pressure drop in the two downstream legs could be controlled to remain relatively equal. This solution presents several problems. First, the maximum pressure reached to provide the necessary flow split accuracy can be very high. This can interfere with other pump features such as an occlusion alarm, and can cause sealing difficulties. Second, if the pump has a user selectable flow rate, the size of the glass orifice must be fixed to work with the lowest possible flow rate. This aggravates the maximum pressure problem should the pump be used at its highest flow rates.
The present invention is aimed at one or more of the problems set forth above.